Polyimide is well known as a separation membrane material which is excellent in heat resistance, chemical resistance and other properties, since polyimide has a high glass transition point and a rigid molecular chain structure. Separation membranes using various kinds of polyimides have been disclosed. For example, U.S. Pat. No. 4,959,151 discloses aromatic polyimide using biphenyltetracarboxylic acid dianhydride. Published Unexamined Japanese Patent Application (JPA) No. 5-7749, U.S. Pat. No. 5,042,992 and others disclose fluorine-containing aromatic polyimide. Polyimides using aliphatic and alicyclic tetracarboxylic acid dianhydride are disclosed in U.S. Pat. Nos. 4,964,887 and 4,988,371.
Polyquinazolone polymers are also developed in order to improve the performance of polyimides (see Published Examined JPA No. 61-51934, Published Unexamined JPA No. 60-75310, and Journal of Polymer Science Vol. 60, 1962). A polyquinazolone polymer material is excellent in chemical and heat resistance, mechanical strength, and film-forming property. However, permeability of such a conventional polyquinazolone polymer was too low to be used for a separation membrane, and thus, such a separation membrane did not reach the level of a practical separation performance.
Poly(dimethylsiloxane), which is a highly permeable membrane material, has an oxygen permeability coefficient of about 10.sup.-8 cm.sup.3 (STP)cm/cm.sup.2 /sec/cmHg!, and is the best of the known polymer membranes. This membrane, however, is inferior in separability, and unpractical in view of its equipment and cost. This membrane must be made comparatively thick since it is not mechanically strong. Therefore, permeation speed cannot be improved by providing a thinner membrane. Though Published Examined JPA No. 47-51715 discloses an oxygen enrichment membrane comprising poly(vinyl trimethylsilane) for a highly permeable membrane material, the membrane is inferior in chemical resistance and it easily deteriorates due to air pollutants, pump oil etc., and its heat resistance is not satisfactory either.
As mentioned above, a membrane comprising a polyquinazolone polymer has chemical resistance, heat resistance, mechanical strength and film-forming property as much as a polyimide separation membrane has. However, a conventional polyquinazolone polymer does not have properties applicable for a separation membrane. That is, its permeability is too low for a practical use.